1. Field of the Invention
The present invention relates to a doped semiconductor material and, in particular, to a doped semiconductor material having a good electrical conductivity and a large forbidden bandgap. The present invention also relates to a method of manufacturing such a doped semiconductor material, and to a semiconductor device incorporating a layer of this material.
2. Description of the Related Art
One well-known compound semiconductor system is the (Al,Ga,In)P system. A compound belonging to the (Al,Ga,In)P system has the general formula (AlxGa1xe2x88x92x)1xe2x88x92yInyP where both x and y are between 0 and 1.
The (Al,Ga,In)P system is widely used in the fabrication of semiconductor layer structures including, for example, optoelectronic devices such as semiconductor laser devices. One advantage of this semiconductor system is that it is lattice-matched to a gallium arsenide substrate when the indium mole fraction, y, is equal to 0.48.
As is well-known, compound semiconductors such as (Al,Ga,In)P can be xe2x80x9cdopedxe2x80x9d by intentionally introducing impurities into the semiconductor. These intentional impurities, known as xe2x80x9cdopantsxe2x80x9d, generate free charge carriers in the semiconductor material, and thus change its electrical properties. In a xe2x80x9cn-dopedxe2x80x9d semiconductor material the majority of the free charge carriers are electrons, whereas in a xe2x80x9cp-dopedxe2x80x9d material the majority of the free charge carriers are holes.
Laser devices or laser diodes (LDs) fabricated in the (Ga,Al,In)P system which emit light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer systems. For example, it is envisaged that the Digital Video Disc (DVD) system will employ a 635 nm-650 nm wavelength LD capable of delivering 5 mW power output up to a temperature of 60xc2x0 C. The next generation of semiconductor lasers will need an even greater maximum power output up to the same or higher (e.g. 70xc2x0 C.) operating temperatures.
FIG. 1 is a schematic view of a semiconductor laser device. The laser device 1 consists of a substrate 2, on which is disposed, in sequence, an n-doped cladding layer 3, a waveguide 4, an active region 5, another waveguide 6, and a p-doped cladding layer 7. In this laser device, light is generated in the active region 5. Light generated in the active region 5 is confined in the vertical direction in FIG. 1 by the waveguides 4, 6. This is done, for example, by ensuring that the refractive index of the waveguides 4, 6 is greater than the refractive index of the active region 5.
The substrate 2 primarily serves to provide mechanical strength for the laser diode. The laser diode is produced by depositing the layer 3-7 sequentially on the substrate 2. This can be done in principle by any conventional semiconductor growth technique, although it is preferred to use MBE (Molecular Beam Epitaxy) or CVD (Chemical Vapour Deposition) since these methods produce materials having high purity and well-defined geometry.
The upper and lower cladding layers 3, 7 serve to confine carriers within the active region. It is therefore necessary for the cladding layers 3, 7 to have a greater forbidden bandgap than the active region 5. The efficiency of the confinement of carriers in the active region improves as the difference in bandgap between the cladding layers and the active region increases, so that it is desirable to maximise the difference between the bandgap of the cladding layers and the bandgap of the active layer.
A principal limitation of current (Al,Ga,In)P LDs is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperatures. It is generally believed that this is caused by electron leakage from the active layer of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
When a laser diode having the structure shown in FIG. 1 is fabricated in the (Al,Ga,In)P system, in one example, the active region 5 is a (Ga,In)P active region, the optical guiding layers 4,6 are (Al0.5Ga0.5)0.52In0.48P, and the cladding layers 3, 7 are (Al0.7Ga0.3)0.52In0.48P. Typical ranges for the thicknesses of the layers are 0.5-4 xcexcm for the cladding layers 3, 7, 0.005-1 xcexcm for the waveguides 4,6, and 0.001-1 xcexcm for the active region 5. The cladding layers 3, 7 are doped, with the upper cladding layer being p-type and the lower cladding layer being n-type. Thus, in order to fabricate a laser diode having the structure shown in FIG. 1 in the (Al,Ga,In)P system, it is necessary to produce doped (Al,Ga,In)P layers to serve as the upper and lower cladding layers 3, 7. As indicated above, it is essential that these cladding layers have a greater forbidden bandgap than the active region 5, so as to confine carriers in the active region thereby enabling generation of light to occur, and it is desirable to maximise the difference between the bandgap of the cladding layer and the bandgap of the active layer.
One problem encountered in fabricating a laser diode in the (Al,Ga,In)P system is that it is energetically favourable for an (Al,Ga,In)P material to exhibit atomic ordering. This is disclosed in xe2x80x9cNature and Origin of Atomic Ordering in III-V Semiconductor Alloysxe2x80x9d by A. G. Norman et al., Inst. Phys. Conf. Ser. No. 134, Section 6, pp. 279-290, 1993. It is known that this ordering causes a reduction in the forbidden bandgap of (Al,Ga,In) P materials. For example, a reduction in the forbidden bandgap of Ga0.5In0.5P is disclosed in Applied Physics Letters Vol. 50 (11), 1987, pp. 673-675. Atomic ordering in the (Al,Ga,In)P system may also cause structural degradation of the material, and thus lead to device failure. It is therefore desirable to reduce the amount of atomic ordering that occurs in the material by as much as possible.
Atomic ordering may occur in any semiconductor system where more than two elements are competing for the same atomic site.
Atomic ordering occurs in the (Al,Ga,In)P system because the size of an In atom differs from the size of either a Ga atom or an Al atom. The presence of two differently sized sites for group III atoms leads to a segregation of In atoms from Al and Ga atoms. This leads to formation of the following:
Anti phase domains;
Anti phase domain boundaries; and
Platelets that are In-rich.
The In-rich platelets locally reduce the forbidden bandgap of the semiconductor layer. The platelets have dimensions typically in the range 10-1,000 xc3x85(1-100 nm).
One approach to reducing atomic ordering is growing an (Al,Ga,In)P layer on an xe2x80x9coff-axisxe2x80x9d substrate. An xe2x80x9coff-axisxe2x80x9d substrate is a substrate in which the surface of the substrate on which the semiconductor layer is grown is slightly misoriented from the crystal plane. For example, rather than the growth surface being the (100) crystal plane, it could be misoriented by a few degrees towards the (111) crystal plane.
Another prior art method of reducing atomic ordering in a semiconductor material is to anneal the material. Annealing a semiconductor material, at a temperature typically in the range of 500xc2x0 C. to 1,000xc2x0 C., reduces atomic ordering by causing redistribution of atoms within the lattice. This redistribution of atoms is promoted if the material contains impurities which become mobile during the annealing, since such impurities will aid the redistribution of atoms.
Although it is relatively straightforward to reduce atomic ordering in an undoped semiconductor material by annealing it, difficulties occur in annealing a doped semiconductor layer such as a doped (Al,Ga,In)P semiconductor layer. One common p-type dopant for (Al,Ga,In)P is beryllium, but beryllium is known to diffuse through a host lattice at temperatures of greater than around 500xc2x0 C. Thus, annealing a beryllium-doped (Al,Ga,In)P layer will cause the beryllium to diffuse through the semiconductor layer. If the doping concentration is not constant throughout the semiconductor layer, then the diffusion of beryllium that occurs during annealing will reduce the sharpness of the doping profile and, moreover, beryllium could diffuse into the active region 5. This diffusion of beryllium will adversely affect the electrical and optical properties of the laser device. This problem is aggravated because the diffusion coefficient increases with the concentration of the dopant. Furthermore, as the beryllium atoms move within the lattice, they will tend to form beryllium precipitates, to cause stacking faults, and to move into interstitial sites thereby reducing the active p-type doping level.
A first aspect of the present invention provides a semiconductor material comprising a first dopant for providing free charge carriers within the semiconductor material and a second dopant for promoting atomic disorder in the semiconductor material, the concentration of the second dopant being substantially uniform over the volume of the semiconductor material.
The first and second dopants are introduced into the semiconductor material during the growth process. Once the semiconductor material has been. grown it is annealed, and when the material is annealed, the second dopant will diffuse through the lattice of the semiconductor material. The second dopant can initially be introduced into the material with a concentration that is non-uniform over the volume of the material or with a concentration that is substantially uniform throughout the material since, even if the concentration of the second dopant was initially not uniform, after the annealing the concentration of the second dopant will be substantially uniform over the volume of the layer of semiconductor material as a result of the diffusion of the second dopant. The diffusion of the second dopant that occurs during the annealing will reduce the atomic ordering of the semiconductor material. This ensures that the forbidden bandgap of the semiconductor material is not reduced because of atomic ordering effects, and ensures that structural degradation of the semiconductor material will not occur. The presence of the first dopant ensures that there is a high, controllable free carrier concentration in the semiconductor material after the annealing step. Provided that the first dopant does not diffuse through the lattice significantly, the doping profile of the first dopant is not changed by the annealing step.
Co-doping of semiconductor materialsxe2x80x94that is, doping a semiconductor material simultaneously with two different dopantsxe2x80x94is known, but it has not previously been used for the reason for which it is used in the present invention.
xe2x80x9cJournal of Applied Physicsxe2x80x9d Vol. 76 (12), 1994, pp. 8189-8191 discloses the co-doping of indium gallium nitride films with silicon and zinc. The silicon acts as an impurity to assist radiative recombination, and thereby increases the output of the light emitting diode.
U.S. Pat. No. 5,231,298 discloses a GaAs layer which is co-doped with a p-type dopant and an indium isovalent, isoelectronic dopant. The indium isovalent, isoelectronic dopant is provided to strain the structure of the lattice in a direction opposite to that caused by the p-type dopant.
U.S. Pat. No. 4,889,830 discloses an indium phosphide layer which is doped with zinc. The diffusion of zinc vapour into the indium phosphide layer is carried out in the presence of cadmium vapour, but at a temperature at which only zinc diffuses into the substrate in significant amounts. This method has been found to preserve the surface morphology of indium phosphide during the zinc diffusion process.
Japanese published patent application Nos. 58-053827, 58-056329 and 62-232122 disclose co-doping of GaAs in order to improve the dopant density in the GaAs layer.
U.S. Pat. No. 5,766,981 discloses a method of forming confining regions in a semiconductor laser device having a superlattice active layer. Once the laser device has been fabricated an impurity which promotes intermixing of atoms is diffused into the laser device from its upper surface, and this impurity causes the active region to inter-mix with the adjacent cladding layers so as to lower the refractive index of the inter-mixed regions. These regions of lower refractive index confine light within the central region of the laser.
U.S. Pat. No. 4,871,690 discloses the provision of disordered regions in the substrate of a semiconductor laser device, for example by implanting impurity atoms into regions of the substrate. The laser device is then grown on the substrate, and defects created in the treated areas of the substrate diffuse upwards into the regions of the layers of the laser device that are above the treated areas of the substrate. This will increase the bandgap of these regions.
EP-A-0 475 618 discloses the fabrication of a GaAs/GaAlAs laser device. Layers of ZnO are disposed on the facets of the laser, and zinc diffuses from these layers into the active layer of the device to form window regions.
The concentrations of the first and second dopants may each be within the range 1017 cmxe2x88x923 to 1021 cmxe2x88x923.
A second aspect of the present invention provides a semiconductor material containing a first dopant for providing free charge carriers in the semiconductor material and a second dopant for providing atomic disorder in the semiconductor material, the first and second dopant having been introduced into the semiconductor material during the growth of the semiconductor material, wherein the concentration of the second dopant varies over the volume of the semiconductor material.
A layer of semiconductor material according to the second aspect of the invention can be annealed in order to produce a layer of semiconductor material according to the first aspect of the invention. Because the second dopant is introduced into the material such that the concentration of the second dopant varies over the volume of the semiconductor material, the amount of diffusion that occurs when the material is annealed will be increased (during an annealing step, the concentration of the second dopant will tend to equalise itself over the volume of the material). Thus if the concentration of the second dopant before the annealing step is not constant, a greater amount of diffusion will be required to occur during the annealing step in order to equalise the concentration of the second dopant over the volume of the material. Increasing the amount of diffusion will further reduce the atomic ordering of the semiconductor material.
The second dopant may be contained within one or more discrete volumes within the semiconductor material. This is one simple way of increasing the amount of diffusion of the second dopant that occurs during annealing. The second dopant may be contained within one or more xcex4-doped layers within the semiconductor material. The concentration of the second dopant within the or each xcex4-doped layer may be within the range 10 2 cmxe2x88x922 to 1014 cm2.
The semiconductor material may be (Al,Ga,In)P. The first dopant may be carbon, and the second dopant may be beryllium, magnesium or zinc. This will produce a p-doped (Al,Ga,In)P layer having, after annealing, a low atomic ordering and thus a large forbidden bandgap. This is suitable for use as, for example, the p-type cladding layer in a laser device such as that shown in FIG. 1.
Carbon is a known p-type dopant for (Al,Ga,In)P, as is disclosed in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 71 (8), 1977, pp. 1095-1097. The diffusion coefficient of carbon in (Al,Ga,In)P will be significantly lower than the diffusion coefficient of beryllium in (Al,Ga,In)P. For example, the diffusion coefficient of carbon in GaAs is about two orders of magnitude lower than that for beryllium in GaAs (xe2x80x9cDoping in III-V Semiconductorsxe2x80x9d by E. F. Schubert, p. 190), and a similar result will hold for the diffusion coefficients in (Al,Ga,In)P. Carbon is therefore much less likely to be redistributed during annealing, so that a high p-type carrier concentration will be maintained. Furthermore, since carbon is not significantly redistributed, the sharpness of the doping profile will be retained. The active p-type doping level will also be retained, since the carbon atoms will not move into interstitial sites. The beryllium atoms will diffuse through the lattice during annealing, and this will reduce the atomic ordering of the semiconductor material. Thus, co-doping with both beryllium and carbon, followed by an annealing step, will result in a material which is disordered and has a high, controllable p-type doping profile.
Magnesium or zinc can be used as a p-type dopant in place of beryllium. Magnesium and zinc atoms will diffuse through the semiconductor lattice when it is annealed, and this diffusion will lead to a reduction in atomic ordering.
Alternatively, the first dopant may be silicon, and the second dopant may be selenium, tin or tellurium. These dopants will produce an n-doped material. The silicon will provide a high level of n-doping, and the selenium, tin or tellurium will diffuse through the semiconductor material when it is annealed so as to reduce the atomic ordering.
A third aspect of the present invention provides a semiconductor device comprising a layer of a semiconductor material as defined above. The device may be a semiconductor laser device, and the layer of the semiconductor material may form a cladding layer of the semiconductor laser device.
The laser device may further include a layer of semiconductor material that is not intentionally doped with the second dopant. This reduces the likelihood that the second dopant will diffuse into the active region of the laser device when the device is annealed.
A fourth aspect of the present invention provides a method of fabricating a semiconductor material, comprising the steps of introducing first and second dopants into the semiconductor material during the growth of the semiconductor material, the first dopant being for providing free charge carriers within the semiconductor material, and the second dopant being for promoting atomic disorder within the semiconductor material.
The second dopant may be introduced into the semiconductor material so that the concentration of the second dopant varies over the volume of the semiconductor material.
The method may comprise the step of annealing the semiconductor material at a temperature at which the second dopant is mobile thereby to make the concentration of the second dopant substantially uniform over the volume of the semiconductor material. The annealing step may be carried out at a temperature within the range 500xc2x0 to 900xc2x0 C.
The second dopant may be introduced only into one or more discrete volumes of the semiconductor material during the growth of the semiconductor material. The semiconductor material may comprise one or more xcex4-doped layers of the second dopant before the annealing step is carried out. The concentration of the second dopant in the or each xcex4-doped layer may be within the range 1012 cmxe2x88x922 to 1014 cmxe2x88x922.
The semiconductor material may be (Al,Ga,In)P. The first dopant may be carbon and the second dopant may be beryllium, magnesium or zinc. Alternatively, the first dopant may be silicon and the second dopant may be selenium, tin or tellurium.
The concentrations of the first and second dopants after the annealing step may each be within the range 1017 cmxe2x88x923 to 1021 cmxe2x88x923.
A fifth aspect of the present invention provides a method of fabricating a semiconductor laser device comprising growing a layer of semiconductor material using a method as defined above.
The method may further comprise the step of growing a spacer layer between the active region of the laser device and the layer of semiconductor material grown using the method of the invention, the spacer layer being not intentionally doped with the second dopant.